1. Field of the Invention
This invention relates to a system and method for determining volume-related parameters, such as the thickness and material properties, of ocular and other biological tissues.
2. Background Art
Glaucoma is an optic neuropathy in which the most important risk factor is ocular hypertension, or elevated intraocular pressure (IOP). IOP is measured easily and reliably with the Goldmann applanation tonometer. Over time, atrophy or loss of the retinal nerve fibers occurs, and when about one half the nerve fibers are lost, characteristic defects develop in the visual field. The diagnosis of glaucoma depends on the presence of these visual field defects, which are permanent, irreversible and, once present, can worsen even with treatment. Eventually, they can coalesce and lead to blindness. The modern method of plotting visual fields, computer-automated perimetry by SITA (Swedish Interactive Threshold Algorithm), is probably near the practical limit of refinement for a test based on patient responses. Because it is subjective, reliability varies with the patient, from excellent to poor.
The optic disc is the beginning of the optic nerve. It is an oval ˜1.8×1.5 mm and its surface contour consists of the neural rim surrounding a depression, or cup. It is ˜1.5 mm thick, and its posterior limit is the lamina cribrosa, a connective tissue grid of pores and beams. The disc (and optic nerve) is made up of about 1.5 million retinal nerve fibers, the unmyelinated axons of the retinal ganglion cells (which die in glaucoma), glia, the central nervous system's supporting cells, and fine blood vessels and capillaries. Loss of the nerve fibers in glaucoma results in a characteristic disc atrophy, which was visualized on examination through the pupil more than 100 years ago. The challenge has long been to detect atrophy before the visual field defects occur, or once field defects are present, to detect slight additional atrophy of disease progression which indicates inadequate treatment. It is not easy to detect change in the contour of the disc surface by direct visualization or photographically because of the paucity of detail on its surface and the highly light-scattering nature of this tissue, like a bundle of optical fibers. Nevertheless, after almost 50 years, the gold standard for determining the status of the optic disc in glaucoma is still the subjective qualitative assessment of stereo photographs obtained with a fundus camera.
The fundus camera is a telescopic system. During the 1970s, efforts to measure the surface contour of the optic disc by applying photogrammetric techniques to photographic images obtained with the fundus camera were unsuccessful. The goal was not significantly advanced using the photo slit lamp, which is a microscopic system to photograph the disc with patterns projected onto its surface (Cohan B E, Multiple-slit illumination of the optic disc, Arch Ophthalmol 1978, 96:497-500; Graebel W P et al., Quantitating Human Optic Disc Topography, Applications of Human Biostereometrics (NATO) 1978, SPIE 166:263-267; Cohan B E et al., Comparison of photo slit lamp and fundus camera photography of the optic disc, Arch Ophthalmol 1979, 97:1462-4).
In the last decade, three imaging technologies have been developed and each has been the target of over 100 research studies. OCT (optical coherent tomography) and GDx (scanning laser polarimetry) have as their target the nerve fiber layer of the retina but is not currently clinically applied to the disc. HRT (Heidelberg Retinal Tomography or scanning laser ophthalmoscopy) provides images of the disc at several depths to assess its surface contour. The common current opinion of glaucoma authorities on the utility of these instruments was clearly expressed by a professor who said he doesn't use any of them clinically. “None of them have been proven superior to optic disc photos and visual field analysis for diagnosing glaucoma or following progression. The American Academy of Ophthalmology hasn't recommended that you follow your patients with one particular technology, for the same reason.”—Donald Budenz, M. D. Bascom Palmer Eye Institute, University of Miami; Ophthalmology Management, August 2003.